/*
 * Copyright 2016 The Cartographer Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef CARTOGRAPHER_COMMON_THREAD_POOL_H_
#define CARTOGRAPHER_COMMON_THREAD_POOL_H_

#include <deque>
#include <functional>
#include <memory>
#include <thread>
#include <vector>

#include "absl/container/flat_hash_map.h"
#include "absl/synchronization/mutex.h"
#include "cartographer/common/task.h"

namespace cartographer
{
    namespace common
    {

        class Task;

        class ThreadPoolInterface
        {
        public:
            ThreadPoolInterface()
            {
            }
            virtual ~ThreadPoolInterface()
            {
            }
            virtual std::weak_ptr<Task> Schedule(std::unique_ptr<Task> task) = 0;

        protected:
            void Execute(Task *task);
            void SetThreadPool(Task *task);

        private:
            friend class Task;

            virtual void NotifyDependenciesCompleted(Task *task) = 0;
        };

        // A fixed number of threads working on tasks. Adding a task does not block.
        // Tasks may be added whether or not their dependencies are completed.
        // When all dependencies of a task are completed, it is queued up for execution
        // in a background thread. The queue must be empty before calling the
        // destructor. The thread pool will then wait for the currently executing work
        // items to finish and then destroy the threads.
        class ThreadPool : public ThreadPoolInterface
        {
        public:
            explicit ThreadPool(int num_threads);
            ~ThreadPool();

            ThreadPool(const ThreadPool &) = delete;
            ThreadPool &operator=(const ThreadPool &) = delete;

            // When the returned weak pointer is expired, 'task' has certainly completed,
            // so dependants no longer need to add it as a dependency.
            std::weak_ptr<Task> Schedule(std::unique_ptr<Task> task)
                LOCKS_EXCLUDED(mutex_) override;

        private:
            void DoWork();

            void NotifyDependenciesCompleted(Task *task) LOCKS_EXCLUDED(mutex_) override;

            absl::Mutex mutex_;
            bool running_ GUARDED_BY(mutex_) = true;
            std::vector<std::thread> pool_ GUARDED_BY(mutex_);
            std::deque<std::shared_ptr<Task>> task_queue_ GUARDED_BY(mutex_);
            absl::flat_hash_map<Task *, std::shared_ptr<Task>> tasks_not_ready_ GUARDED_BY(mutex_);
        };

    } // namespace common
} // namespace cartographer

#endif // CARTOGRAPHER_COMMON_THREAD_POOL_H_
